1. Field of the Invention
The present invention relates to a rack transport control apparatus for manufacturing lines, and more particularly, to a rack transport control apparatus for a manufacturing line including a series of processes for producing a plurality of types of articles, which apparatus controls the flow of racks carrying semimanufactured articles to and from the individual processes.
2. Description of the Related Art
In recent years, manufacturing systems aided by a host computer have been applied to manufacturing lines for producing a plurality of types of articles in small quantities, and automated manufacturing Increased in flexibility and efficiency is under development.
An example of such manufacturing lines is illustrated in FIG. 1. This manufacturing line comprises a plurality of processes 10, 20 and 30, and a storage 100 for temporarily keeping articles half-finished by the processes. The storage 100 includes buffers associated with the respective processes 10, 20 and 30. The buffers are provided to reduce the stagnation of semimanufactured articles arising from the difference in time required by the respective processes, and are used to temporarily keep semimanufactured articles to be conveyed to the corresponding processes. Manufacturing equipments 11 and 12 are provided for the process 10, a manufacturing equipment 21 for time process 20, and manufacturing equipments 31 and 82 for the process 30.
In the manufacturing line constructed as above, a plurality of semimanufactured articles, such as printed wiring boards to which a large number of electronic parts are mounted, are contained in racks according to their kinds, and each rack Is conveyed by a transport vehicle 200 to the process to which the semimanufactured articles contained therein are to be directed. Semimanufactured articles are finished when they have passed through the required processes. Each rack comprises a box for containing a plurality of semimanufactured articles of the same kind.
In this type of manufacturing lines, a plurality of types of articles are concurrently manufactured; namely, different types of articles are passed through different processes, requiring different tasks and times in the individual processes. Thus, there is a possibility that a large number of semimanufactured articles may be contained in any of the buffers associated with time processes 10, 20 and 30. To prevent this, a time management method has been conventionally employed. Specifically, when introducing new semimanufactured articles into the inlet of the manufacturing line, the time period these articles would require up to the end of the final process is estimated by simulation on the basis of the kinds of the semifinished articles then involved in the line, the storage state, etc. The new articles are introduced into the line only when it is ascertained that the estimated time is within a predetermined time, thus administering the time required up to the final process.
However, managing only the total time required up to the final process is not enough and unable to cope with the limitation on time elapsed between specific processes of the manufacturing line. For example, in an SMT (surface mount technology) unit assembly line for mounting electronic parts to printed wiring boards, once solder paste is applied, soldering must be completed before the applied solder paste dries. Thus, there is a limitation on the wait time between the end of the solder paste application process and the start of the soldering process. Even though the total time required up to the final process is within the predetermined time, the wait time may be so long that the solder paste dries. In such cases, a large number of defective articles may result.
Further, even if the total time is estimated by simulation and it is ascertained that the estimated time is within the predetermined time, trouble may occur in any of the equipments 11 to 32 during actual processing after semimanufactured articles are introduced into the inlet of the manufacturing line. In such cases, the processes will not be carried out in the simulated manner, and there still is a possibility that a large number of semimanufactured articles may be contained in the buffers.